The present invention relates to a method and apparatus for connecting a device to a bus carrying power and a signal, and more particularly to a method and apparatus for connecting a computer peripheral bus or a consumer electronics bus carrying power and a signal to a peripheral device, such as a speaker, that is powered in whole or in part by the bus and that uses the signal carried by the bus.
The universal serial bus ("USB") holds great promise for improving the ease with which computer peripherals, such as keyboards and speakers, can be attached to personal computers. The USB standard is specified in a series of documents available via the World Wide Web at http:.backslash..backslash.www.usb.org.
One important aspect of the USB standard is management of devices that draw power from the USB. The USB standard defines two types of devices, low power devices that draw 100 mA of bus current and high power devices that draw 500 mA or less of the bus current.
The USB standard supports attaching to a USB both devices that draw power from the USB and devices that are powered by other, external, sources of power such as batteries or line voltage transformers.
The importance of power management and the relatively low power drain supported by the USB standard has led to two separate approaches for USB devices that may draw more than the permitted power. One is to create a "powered" hub that draws power from an external source to support more peripheral devices that the USB can power alone. The other is to provide an external power supply for the particular USB device.
While it is generally adequate to use powered hubs (with their external power supplies) with other USB peripheral devices (with their external power supplies), the need for external power supplies makes the connecting and the operating the devices more complex than if the device were powered from the USB alone. Moreover, with external power supplies such as batteries or wall transformers, there is the disadvantage of loss of power to the peripheral due to the battery draining or the wall transformer becoming dislodged from the wall. Wall transformers are also often unattractive (hence the nickname "wall wart") and have cords that can become entangled. Batteries can be expensive to replace. Batteries can also be hassles to change.
All of these disadvantages of external powered computer peripheral devices are particularly acute for users of speakers. Speaker users are particularly sensitized to issues of speaker performance and ease of operation. Speakers typically operate in pairs (to provide for the option of stereo sound), so a speaker "problem" is usually a double problem. Computer speakers may be embedded in another computer peripheral (such as a desk top keyboard or monitor), in which case they are generally not usable with other computers, and may still require the use of external power supplies and additional cabling for operation with the initially intended computer. Computer speakers may be attached to a personal computer via cables to allow wider separation for optimum stereo separation, in which case an extra cable for the wall transformer is an unwelcome complication.
One great appeal of USB peripherals, such as USB speakers, stems from their "Plug and Play" installation and their operational behavior. With Plug and Play, installing new peripherals does not require disassembly of the computer case to install special cards or change jumper/switch settings of existing cards and does not require knowledge of interrupt request and DMA settings. The new peripheral identifies itself upon interrogation by the host computer system. The USB protocols, correctly implemented, assure absence of device conflicts.
It is therefore unfortunate that existing implementations of USB speakers and many other USB peripheral devices require the devices to be "self-powered" (i.e., not powered by the USB) due to the limited power available from USB ports. Self powered USB devices, by definition, have the added complication of batteries or transformers or other means of supplying external power to their associated USB device. Yet most of these self-powered USB devices do not require average power in excess of the continuous power available from low-power or high-power USB ports. In particular, while speakers reproducing music, typical speech, or game sound effects require large peak powers, they require far less average power, even if their power requirements are averaged over a time scale of the order of a few tenths of a second. Other peripheral devices with similar power demand characteristics include printers, infrared data links, scanners and other devices in which electromechanical or electro-optical transduction is, or can be, discontinuous and of a low duty cycle.
There is therefore a need for a device that provides a high intermittent peak power output while simultaneously limiting its current input to an amount at or below the maximum current input allowed by the USB standard or by the standard of any other bus (such as other serial buses, like the serial bus defined by IEEE-1394, or parallel buses, like the Small Computer Systems Interface or SCSI bus).